In certain industries, heat exchangers are required to be opened weekly or daily to inspect the heat transfer plates. This process can require the removal of one or more plates for closer inspection or cleaning.
Traditionally, one of the end plates, commonly referred to as the head, is fixed and the other end plate, commonly referred to as the follower, is moveable towards the head to close the heat exchanger and is movable away from the head to open the heat exchanger.
Heat exchangers of this type are well known and typically include at least two spindles carrying nuts that can be rotated to urge the follower towards the head. Manual rotation of the nuts can result in uneven closure forces being applied to the package of heat transfer plates by the follower. This can lead to incomplete sealing between the heat transfer plates giving rise to leaks. This in turn may lead to contamination of a product, for example milk, by coolant.
APV Products previously developed a heat exchanger having a powered closure system first available in the USA in 1987 and known as a CR-5 plate heat exchanger. This heat exchanger is shown in FIG. 1 and includes a support frame 1 for a plate pack 2 located between a fixed head 3 at one end of the frame 1 and a movable follower 4. As shown, the plate pack 2 includes groups of heat transfer plates 5, 6 separated by connector grids 7 and divider plates 8. The plate pack 2 is located and supported between horizontal upper and lower beams 9, 10 extending between the head 3 and a drive housing 11 at the other end of the frame 1.
The follower 4 is arranged between the beams 9, 10 and is movable towards the head 3 by a pair of jack screws 12, 13 extending between the follower 4 and the drive housing 11. The jack screws 12, 13 are operable synchronously by a drive mechanism (not shown) located within the drive housing 11. The drive mechanism includes an electric motor, hydraulic pump and hydraulic motor to drive synchronously two coaxial drive sprockets each connected to a driven sprocket by a separate flexible drive chain. The driven sprockets are coupled to two jack nuts that rotate and thereby move the jack screws 12, 13 and the output from the motor is reversible for rotating the driven sprockets in either one of two opposed directions.
In this way, rotation of the sprockets in one direction simultaneously and synchronously extends the jack screws 12, 13 and rotation of the sprockets in the opposite direction simultaneously and synchronously retracts the jack screws 12, 13. As a result, extending the jack screws 12, 13 pushes the follower 4 towards the head 3 to clamp the plate pack 2 between the head 3 and follower 4. Retracting the jack screws 12, 13 permits the follower 4 to move away from the head 3 to release the plate pack 2 for inspection.
Although the powered system avoids some problems associated with manual operation of the closure system, the jack screws 12, 13 are loaded in compression when the heat exchanger is closed and there is an inherent limitation in the length of the jack screws 12, 13 that can be employed. Thus, only a certain number of plates can be installed without increasing the diameter of the jack screws 12, 13 and plate quantity requirements in certain industries already exceed the limitations of this design. In addition, the drive housing 11 has to be sized to accept the full compressive and hydraulic loads associated with closing and pressurizing the heat exchanger.
FIGS. 2 and 3 show heat exchangers with powered closure systems as disclosed in U.S. Pat. No. 5,462,112 to Johansson, issued Oct. 31, 1995.
The closure system shown in FIG. 2 is similar to that employed in the CR-5 plate heat exchanger described above with reference to FIG. 1 and has four bolts 20-23 extending between the follower 24 and a frame plate 25 supporting a motor 26. The bolts 20-23 engage at one end nuts 27, 28 (two only shown) fixed to the follower 24 and at the other end nuts 29-32 rotatably supported on the frame plate 25. The nuts 29-32 are synchronously rotatable by the motor 26 via a flexible endless drive belt 33. In this way, the bolts 20-23 are axially extendable to push the follower 24 towards fixed head 34 to clamp the plate pack 35 by rotation of the nuts 29-32 in one direction. Rotation of the nuts 29-32 in the opposite direction moves the follower 24 away from the head 34 to release the plate pack 35. With this arrangement, the bolts 20-23 are loaded in compression when the heat exchanger is closed and this system therefore suffers from the same structural limitations and disadvantages as the system shown in FIG. 1.
The closure system shown in FIG. 3 has four bolts 50, 51 (two only shown) that are loaded in tension when the heat exchanger is closed. Two bolts 50, 51 extend between the fixed head 52 and the movable follower 53 on one side of the plate pack 54 and the other two bolts (not shown) extend between the fixed head 52 and follower 53 on the other side of the plate pack 53. The drive mechanism is mounted on the fixed head 52 and includes a motor 55 for simultaneously and synchronously rotating all the bolts 50, 51 (as well as the two bolts not shown) via an endless flexible drive belt (not shown). Each bolt 50, 51 engages a nut 56, 57 (two only shown) that is prevented from rotating and separating axially from the follower 53.
In this way, rotation of the bolts 50, 51 causes the nuts 56, 57 to move axially along the bolts 50, 51 carrying with them the follower 53. As a result, the follower 53 is pulled towards the fixed head 52 by rotation of the bolts 50, 51 in one direction to close the heat exchanger. Rotation of the bolts 50, 51 in the opposite direction pushes the follower 53 away from the fixed head 52 to open the heat exchanger.
As can be seen, with this arrangement, access to the plate pack 54 is restricted by the bolts 50, 51 (and the two not shown) on each side and by the upper and lower beams 58, 59 connecting the fixed head 52 to the plate 60 at the other end of the support frame. Accordingly, if it is desired to remove one or more plates 61 from the heat exchanger, at least two of the bolts 50, 51 on one side of the plate pack 54 must first be removed to provide access to withdraw the plates 61 sideways. On heat exchangers with large plate packs 54 and therefore longer and heavier bolts 50, 51, such a task can exceed the strength of one person and thereby necessitate the use of further personnel or even mechanical handling equipment.
Furthermore, before removal of the bolts 50, 51, the drive belt first has to be completely removed from the driving mechanism. Because the drive belt is under tension, the tensioner mechanism must be relaxed further extending the time and effort required to access the plate pack 54. Such removal of the drive belt is highly unconventional for normal machine operation and imposes a complexity that goes beyond the expected expertise of general heat exchanger operators.
Moreover, replacement of the bolts 50, 51 and the drive belt may require the exact relative alignment of each driven coupling to the bolts 50, 51 to ensure parallel movement of the follower 53 towards and away from the fixed head 52.